Apparatus and methods for low overhead highly reliable determination of rotational position of a magnetic storage medium

ABSTRACT

A highly reliable, low overhead method for determining a rotational position of a magnetic storage medium that is divided into a plurality of servo sections is provided. The inventive method includes defining a predetermined bit pattern that corresponds to a known position of the magnetic storage medium. A rotational position indicator bit is associated with each servo sector such that a known bit of the predetermined bit pattern is associated with the known position of the magnetic storage medium. A current rotational position indicator bit is read from the magnetic storage medium and a rotational position bit sequence comprising the current rotational position indicator bit is formed. The rotational position bit sequence is compared with the predetermined bit pattern and, based on the comparison, whether the rotational position of the magnetic medium is the known rotational position can be determined.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.09/338,098, filed Jun. 23, 1999, the contents of which are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to magnetic storage media and, moreparticularly, to apparatus and methods for low overhead, highly reliabledetermination of the rotational position of such a storage system.

BACKGROUND OF THE INVENTION

Media for recording and reading information are ubiquitous in electronicequipment such as computers, digital cameras, and the like. One type ofsuch media is a flexible data storage disk having a magnetic recordinglayer disposed over a non-magnetic substrate layer. Examples of theseflexible disks include the type commonly referred to as a “floppy disk,”a ZIP disk (manufactured by the assignee of the present invention), andthe like.

Magnetic disks typically have an “embedded servo,” wherein data regionshaving data and control signal regions having servo information forcontrolling the position of the magnetic head, are recorded alternatelyto constitute a recording track. During a process commonly known as“servo-writing,” servo information is embedded into the magnetic layerof the magnetic disk. The servo information typically definessubstantially concentric, circular tracks. Often, servo information isembedded in a sequence of quadrature servo patterns. Each patterntypically comprises four servo pulses, or bursts, each of which isoffset from the neighboring servo burst. Such an embedded servo typedisk has an advantage with respect to data recording density, ascompared with magnetic disk apparatus in which the track on which thedata are recorded and the tracks on which the servo signals are recordedare formed separately on the magnetic disk.

In a conventional embedded servo type magnetic disk apparatus, aread/write control signal, namely the index sector pulse (ISP) signal,is produced in response to the servo information from the control signalof the track, and the read data are transferred to an external unit(e.g. a host computer) in response to the generation of the ISP signal(concerning the “data transfer” operation).

In the conventional magnetic disk apparatus employing embedded servo,one index sector pulse signal functioning as the base point for theread/write control is outputted in response to the servo informationwhich has been read out from the servo (control signal) region of therecorded disk plane. That is to say, in response to signal edges of theservo information S_(n), S_((n+1)) and S_((n+2)), the index sector pulsesignals ISP_(n), ISP_((n+1)), and ISP_((n+2)) are formed. In response tothese ISP signals, the read/write controls for the corresponding dataID_(n), ID_((n+1)), ID_((n+2)) are performed. In other words, a singleread/write operation is carried out with respect to a single servoregion. This process is described in greater detail in U.S. Pat. No.5,313,340.

To determine head location relative to a track centerline, a head ortransducer measures the signal from each burst. A position error signal(“PES”) is determined by comparing the amplitude of the signals readfrom neighboring bursts. The PES is proportional to the differencebetween the signal amplitudes of the neighboring bursts, divided by thesum of their signal amplitudes. Thus, the PES represents the offsetdistance between the head and track centerline as defined by the servoinformation embedded in the disk. The PES is then used as part of aclosed loop servo system to correct the position of the head withrespect to the track.

There are known methods for determining the rotational position of amagnetic medium but these methods typically require a relativelysignificant amount of overhead to be reliable. That is, these methodstypically require a relatively high number of bits on the mediumdedicated to rotational positional determination. Thus, there is a needin the art for a highly reliable, low overhead method for determiningthe rotational position of a magnetic storage medium.

SUMMARY OF THE INVENTION

A highly reliable, low overhead method for determining a rotationalposition of a magnetic storage medium that is divided into a pluralityof servo sections includes defining a predetermined bit pattern thatcorresponds to a known position of the magnetic storage medium. Arotational position indicator bit is associated with each servo sectionsuch that a known bit of the predetermined bit pattern is associatedwith the known position of the magnetic storage medium. A currentrotational position indicator bit is read from the magnetic storagemedium and a rotational position bit sequence comprising the currentrotational position indicator bit is formed. A shift register is used toform the rotational position bit sequence by storing a preexisting bitsequence, eliminating the most significant bit of the preexisting bitsequence to form a temporary bit sequence, and appending the currentrotational position indicator bit to the temporary bit sequence. Therotational position bit sequence is compared with the predetermined bitpattern and, based on the comparison, whether the position of themagnetic medium is the known position can be determined.

To determine whether the magnetic medium is at one of several knownpositions, a method according to the present invention includes defininga set of predetermined bit patterns wherein each predetermined bitpattern corresponds to one of the several known positions. Therotational position indicator bits are associated with the servo regionssuch that a known bit of each predetermined bit pattern is associatedwith the corresponding position of the magnetic storage medium. Therotational position bit sequence is compared with each predetermined bitpattern within the set and, based on the comparison, whether theposition of the magnetic medium is one of the known positions can bedetermined.

The length of the bit patterns, as well as the patterns themselves aredefined based on an allowable number of bit errors. To determine the bitpatterns, a required distance between bit patterns is calculated basedon the allowable number of bit errors. The required distance is onegreater than the number of allowable bit errors. The set ofpredetermined patterns is then determined such that the pattern set hasa distance of at least the required distance. A set of four, nine-bitpatterns (001110101, 010110011, 101001101, 101111011) is provided for anapplication wherein the number of allowable bit errors is two.

A data storage medium according to the present invention has a servopattern comprising a plurality of servo sections. Each said servosection represents a portion of the magnetic storage medium. A singlerotational position indicator bit is associated with each said servosection. The servo pattern is written onto the storage medium such thatthe rotational position indicator bits of adjacent servo sections form apredetermined bit pattern that is associated with a known position ofthe magnetic storage medium.

The data storage medium can then be used in a method as described abovefor determining whether the magnetic storage medium is at one of aplurality of known positions. First, a current rotational positionindicator bit is read from the storage medium. A rotational position bitsequence comprising the current rotational position indicator bit isformed from the rotational position indicator bits of the adjacentconsecutive servo sections. The rotational position bit sequence iscompared with the predetermined bit patterns and, based on thecomparison, whether the data storage medium is at one of the knownpositions can be determined.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of the present invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings. For thepurpose of illustrating the invention, there is shown in the drawingsembodiments that are presently preferred, it being understood, however,that the invention is not limited to the specific methods andinstrumentalities disclosed. In the drawings:

FIG. 1 shows the format of a prior art embedded servo system.

FIG. 2 shows the format of a representative servo section that can beused in accordance with the present invention.

FIG. 3 shows a magnetic storage medium having thereon a servo patternaccording to the present invention.

FIG. 4 is a schematic block diagram of a control circuit for performingread and write operations on a magnetic storage medium formattedaccording to the present invention.

FIG. 5 is a flowchart of a method for determining a position of amagnetic storage medium divided into a plurality of servo sections.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows the format of a prior art embedded servo system. FIG. 1also shows a timing chart representing both a data stream read out froma magnetic disk, and also an index sector signal comprising index sectorpulses ISP_(n), ISP_((n+1)) and ISP_((n+2)) that is formed from servoinformation S_(n), S_((n+1)), and S_((n+2)) included within the datastream.

FIG. 2 shows the format of a representative servo section 30 that can beused in accordance with the present invention. As shown, servo section30 comprises three regions: a servo region 10, an identification (ID)region 11, and a data region 12. Preferably, servo region 10 comprises awrite-read (W-R) and speed field 14, an address mark (AM) field 15, aninformation field 16, and a position error signal (PES) field 17. W-Rand speed field 14 allows time for the drive electronics to switch fromwrite to read. AM field 15 is an asynchronous, absolute timing referencethat identifies the beginning of the servo region and provides the basisfor locating the other fields. Information field 16 includes informationsuch as cylinder number, index sector indication, etc. PES field 17includes information used to determine the track position of therecording head.

ID region 11 comprises a read-write (R-W) and speed field 18, a VCOsynchronization (VCO sync) field 19, an encoder/decoder (ENDEC) flushfield 20, a sync byte 21, and an identification (ID) and cyclicredundancy check (CRC) field 22. R-W and speed field 18 allows the timeneeded to ensure that nothing in servo region 10 is overwritten, andthat sufficient time is provided for the write current to rise to itsfull value. VCO sync field 19 is required to give the variable frequencyread clock sufficient time to phase lock to ID and CRC field 22. ENDECflush field 20 indicates the number of bits the read channel decodermust receive in order to put it into a known state called ENDEC flush.Sync byte 21 indicates the sync byte needed to align the read bytes oncurrent byte boundaries. ID and CRC field 22 includes as the ID portiona sector identifier and bad sector flag and as the CRC portion a cyclicredundancy check for errors in the reading of the ID.

In data region 12, fields 23-26 correspond to fields 18-21,respectively. The function of sync byte field 26, however, is to tellthe controller when VCO synch field 24 and ENDEC flush field 25 end andthe data, which is contained in data and ECC field 27, begins. Data andECC field 27 stores the user data together with the error correctioncode.

A more detailed description of a conventional servo section is providedin U.S. Pat. No. 5,285,327, issued to Hetzler, et al.

According to the present invention, information field 16 in servo region10 includes a single rotational position indicator bit, which is used todetermine whether the magnetic medium is in one of several knownpositions. These known positions are commonly called “rotationalposition indexes.” As will be described in greater detail below, themethods and apparatus of the present invention require only one bit perservo section to determine whether the magnetic medium is at arotational position index. Consequently, these methods significantlyreduce the overhead needed to perform this function.

FIG. 3 shows a magnetic storage medium 50 having a servo patternthereon. As shown in FIG. 3, magnetic storage medium 50 is a disk,although the present invention can be embodied in other magnetic storagemedia, such as magnetic tape, for example. Magnetic storage medium 50 isformatted into a plurality of tracks or bands A-D. Although four bandsA-D are shown in FIG. 3, magnetic storage medium 50 can be formattedwith any number of bands. According to the present invention, the servopattern on magnetic storage medium 50 comprises a plurality of servosectors 30 (on a disk, servo sections are more commonly known as servosectors).

FIG. 3 depicts a disk 50 formatted as banded sectors. Servo sectors 30in each respective band A-D are written at equally circumferentiallyspaced intervals and are sampled (i.e., read) during seek, settle, andtrack following operations. An allowable number of servo sectors perrevolution and the lengths of associated data regions 51 on disk 50 iscalculated such that each of the equally spaced servo sectors 30 on agiven track is located within a data region 51 or immediately after anindex mark 52. In a preferred embodiment of the invention, magneticstorage medium 50 is a disk wherein each band A-D is divided into 80equally sized servo sectors 30. As described above, each servo sector 30has a servo region 10 comprising an information field 16 that includes arotational position indicator bit (see FIG. 2).

A plurality of index marks 52 are written at preselected, knownpositions on magnetic storage medium 50. In a preferred embodiment, fourindex marks 52 are present in each band A-D, although there can be feweror more. Preferably, the index marks 52 are written at equallycircumferentially spaced intervals around the disk 50, and the indexmarks 52 in each band A-D are at the same rotational position as thecorresponding index marks 52 in each of the other bands A-D. Each indexmark 52 is associated with a corresponding, predetermined N-bit pattern.For reasons that will be described in greater detail below, thepredetermined bit pattern is preferably a nine bit pattern, and isselected from the group consisting of 001110101, 010110011, 101001101,and 101111011. These patterns were selected such that during thepositional index search process up to two rotational position indicatorbits may be in error without a false index position being obtained.

For each index mark 52, a known bit of the corresponding predeterminedbit pattern is associated with the index mark 52. That is, therotational position indicator bit in a first servo sector (e.g., theservo sector nearest to the index mark) is set to the value of the knownbit. Thereafter, the rotational position indicator bits in the servosectors adjacent to the first servo sector are set to the values of theadjacent bits in the predetermined bit pattern corresponding to theindex mark 52.

For example, the rotational position indicator bit in the informationfield of servo sector 30-0, which is nearest to index mark 52, ispreferably the least significant bit of the predetermined bit patternassociated with index mark 52. For example, if the predetermined bitpattern associated with index mark 52 is 101111011, then the rotationalposition indicator bit in servo sector 30-0 is a “1.” In this way, aknown bit of a predetermined bit pattern is associated with a knownposition of the magnetic medium. The remaining bits of the predeterminedbit pattern are associated with the adjacent servo regions from leastsignificant to most significant. That is, for the example given, therotational position indicator bit in the first adjacent servo region30-1 would be a “1”, the rotational position indicator bit of the secondadjacent servo region 30-2 would be a “0”, etc. Accordingly, therotational position indicator bit of the eighth adjacent servo region30-8 would be a “1.” In a preferred embodiment, the remaining rotationalposition indicator bits are set to a value of zero. That is, therotational position indicator bit is a “0” for any servo sector 30 thatdoes not have a bit from one of the patterns associated with it (e.g.,servo sectors 30-9 and 30-10 ).

This pattern is written for the corresponding servo sectors in each ofthe bands A-D. That is, the rotational position indicator bit associatedwith servo sector 30A is the same as the rotational position indicatorbit associated with servo sectors 30B, 30C, and 30D. Thus, in each bandA-D, the nine bit pattern 101111011 can be used to determine whether themagnetic storage medium 50 is positioned at index mark 52. In this way,a known rotational position of the magnetic medium can be identifiedregardless of which band is currently being read.

Each of the remaining patterns is distributed about the magnetic mediumin the manner described above for the first pattern, with the leastsignificant bit of each pattern written as the rotational positionindicator bit of the servo region nearest to the corresponding indexmark. Thus, in each band, each of the predetermined bit patterns can beused to determine whether the magnetic storage medium is rotationallypositioned at the index mark that corresponds to that predetermined bitpattern.

FIG. 4 is a schematic block diagram of a control circuit for performingthe read and write operations to determine whether magnetic medium 50 ispositioned at one of the known locations, or index marks. Data 113 isread via a head 111 mounted on an arm 112, and is inputted via aread/write amplifier 114 into a read circuit 115. The data inputted intothe read circuit 115 is classified, or separated into servo information“S” and real data. The separation between the servo information “S” andthe data may be accomplished by employing a known separating circuit.The servo information “S” is inputted into a control logic circuit 116,and further the data is directly inputted into a serial-to-parallelconverter 117. The logic control circuit 116 produces the index sectorpulse signal ISP functioning as a base point for decoding the data inaccordance with the inputted servo information S. The index sector pulsesignal ISP is applied to the serial-to-parallel converter 117. Theserial/parallel converter 117 decodes the data based on the inputtedindex sector pulse signal ISP. In an internal circuit of the controllogic circuit 116, an index sector pulse signal, ISP, is generated whichcan be readily produced from the servo information S.

A writing operation of data series will now be explained. Write data“WD” which has been externally inputted, is converted into a serial dataseries (stream) by way of a parallel-to-serial converter 118 and thenthe converted write data is inputted onto a write circuit 119. The indexsector pulse signals ISP outputted from the control logic circuit 116are used as a starting point. Since the servo information “S” has beenalready written into the track of the disk 50 (note that the data ID hasnot yet been written), the index sector pulse signal ISP is producedbased on the readout servo information “S” similar to the readoperation. Then, the write data “WD” which has been converted intoserial data, is recorded on the magnetic disk 50 via the write circuit119, a read/write amplifier 114, and the head 111. Accordingly, the datarecorded on the magnetic disk 50 is recorded in synchronism with theindex sector pulse signal ISP (ISP-A, ISP-B).

In accordance with the present invention, the current rotationalposition indicator bit, i.e., the rotational position indicator bit readfrom the current servo sector 30, is shifted into the least significantbit position of an N-bit shift register 120 to form a rotationalposition bit sequence as shown in FIG. 4. The contents of this registerare compared with the set of predetermine bit patterns stored in memory121. If comparator 122 determines that one of the comparisons results inan equal, then it is established that magnetic storage medium 50 is atthe known position associated with the predetermined bit pattern thatmatched the positional bit sequence currently in shift register 120.Once rotational position has been established on the disk, for example,a similar set of operations may be performed to verify that rotationalposition has not been lost. It should be understood that shift register120, memory 121, and comparator 122 can be implemented in software,which is currently preferred, or in hardware.

Preferably, the length of the bit patterns, as well as the patternsthemselves, can be defined based on the number of allowable bit errors.For example, the nine-bit patterns provided above were selected suchthat during the rotational position index search process up to tworotational position indicator bits may be in error without a false indexposition being obtained.

The so-called “distance” between two bit patterns is defined to be thenumber of bits that would have to change (e.g., via bit errors) to causethe detection of a valid pattern at an incorrect rotational position. Itis known that the distance between patterns should be at least onegreater than the number of allowable bit errors. In this case, forexample, where the allowable number of bit errors is two, the requireddistance between bit patterns was defined to be at least three bits.

The following procedures can be used to determine the required distance.

Procedure 1.0

This procedure can be used to determine the distance between a “storedpattern” and a “target pattern” if the patterns are not equal, but areof the same length (n).

First, create a new pattern by appending n zero bits to the left and(n−1) zero bits to the right of the “stored pattern” to form an“extended stored pattern” of length (3n−1).

Then, for each starting bit position from 0 to (2n−1) in the “extendedstored pattern”, count the number of bit positions that differ betweenthe corresponding n bits in the “extended stored pattern” and the“target pattern”. The bit at the starting bit position in the “extendedstored pattern” is compared to the bit at bit position 0 in the “targetpattern”. The minimum number from the previous 2n operations is thedistance between the “stored pattern” and the “target pattern”.

Procedure 2.0

To determine the distance between a “stored pattern” and a “targetpattern” if the patterns are equal and of length (n), determine thedistance as if the patterns were not equal (using Procedure 1.0), butignore the count operation when the starting bit position is n. Thenumber of differences when the starting bit position is n will be zerosince this is where the “stored pattern” and the “target pattern” areequal.

Procedure 3.0

This procedure can be used to determine the distance for a set of (m)patterns of length (n).

First, determine the distance for each pattern with the pattern used asboth “stored pattern” and “target pattern” using Procedure 2.0 (mdistances will be found).

Then, determine the distance for each pattern with each pattern used asthe “stored pattern” and every other pattern used as the “targetpattern” using Procedure 1.0 (m²−m distances will be found). The minimumdistance between the first step and the second step is the distance forthe set of patterns.

EXAMPLE

Consider the example wherein the set of patterns consists of 001110101,010110011, 101001101, and 101111011. In this example, therefore, m=4 andn=9.

First, use Procedure 2.0 with “stored pattern”=001110101 and “targetpattern”=001110101. The distance is three.

Extended Stored Number of Pattern 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 1 0 1 00 0 0 0 0 0 0 Differences Start Position =  0 0 0 1 1 1 0 1 0 1 5  1 0 01 1 1 0 1 0 1 5  2 0 0 1 1 1 0 1 0 1 5  3 0 0 1 1 1 0 1 0 1 4  4 0 0 1 11 0 1 0 1 5  5 0 0 1 1 1 0 1 0 1 4  6 0 0 1 1 1 0 1 0 1 6  7 0 0 1 1 1 01 0 1 3 (min)  8 0 0 1 1 1 0 1 0 1 5  9 0 0 1 1 1 0 1 0 1 0 (ignore) 100 0 1 1 1 0 1 0 1 6 11 0 0 1 1 1 0 1 0 1 4 12 0 0 1 1 1 0 1 0 1 7 13 0 01 1 1 0 1 0 1 4 14 0 0 1 1 1 0 1 0 1 5 15 0 0 1 1 1 0 1 0 1 5 16 0 0 1 11 0 1 0 1 6 17 0 0 1 1 1 0 1 0 1 6

Next, use Procedure 1.0 with “stored pattern”=001110101 and “targetpattern”=010110011. Again, the distance is three.

Extended Stored Number of Pattern 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 1 0 1 00 0 0 0 0 0 0 Differences Start Position =  0 0 1 0 1 1 0 0 1 1 5  1 0 10 1 1 0 0 1 1 5  2 0 1 0 1 1 0 0 1 1 5  3 0 1 0 1 1 0 0 1 1 4  4 0 1 0 11 0 0 1 1 3 (min)  5 0 1 0 1 1 0 0 1 1 4  6 0 1 0 1 1 0 0 1 1 6  7 0 1 01 1 0 0 1 1 5  8 0 1 0 1 1 0 0 1 1 3 (min)  9 0 1 0 1 1 0 0 1 1 4 10 0 10 1 1 0 0 1 1 4 11 0 1 0 1 1 0 0 1 1 6 12 0 1 0 1 1 0 0 1 1 5 13 0 1 0 11 0 0 1 1 6 14 0 1 0 1 1 0 0 1 1 3 (min) 15 0 1 0 1 1 0 0 1 1 7 16 0 1 01 1 0 0 1 1 4 17 0 1 0 1 1 0 0 1 1 6

Finally, use Procedure 1.0 or Procedure 2.0 to fill each entry in thefollowing matrix with the distance.

Stored Pattern 001110101 010110011 101001101 101111011 Target Pattern001110101 3 3 3 3 010110011 3 4 3 3 101001101 3 3 3 3 101111011 3 3 3 4

The minimum from the above matrix (i.e., three in this case), is thedistance for this pattern.

Using these procedures many sets of error tolerant patterns can beobtained. The number of patterns in a set, the number of errors allowed,and the number bits in the pattern can all be traded off to best fit theapplication.

If two bit error tolerance is required, the following are examples ofsome of the possibilities:

6 bit pattern 6 sets of 1 pattern 7 bit pattern 31 sets of 1 pattern 8bit pattern 105 sets of 1 pattern 185 sets of 2 patterns 8 sets of 3patterns 9 bit pattern 286 sets of 1 pattern 6,351 sets of 2 patterns20,675 sets of 3 patterns 10,230 sets of 4 patterns 631 sets of 5patterns 4 sets of 6 patterns

If three bit error tolerance is required, the following are examples ofsome of the possibilities:

8 bit pattern 2 sets of 1 pattern 9 bit pattern 42 sets of 1 pattern

As discussed above, a preferred embodiment of a magnetic mediumaccording to the present invention includes four index marks and,consequently, four predefined bit patterns. It was determined that, todefine a set of four bit patterns wherein each pattern is a “distance”of three bits from every other pattern, the bit patterns had to be atleast nine bits in length.

FIG. 5 shows a flowchart of a method 400 according to the presentinvention for determining a rotational position of a magnetic storagemedium divided into a plurality of servo sections. According to theinventive method, at step 402, a predetermined bit pattern is definedthat corresponds to a known position of magnetic storage medium. At step404, a rotational position indicator bit is associated with each servosection such that a known bit of the predetermined bit pattern isassociated with the known position of the magnetic storage medium. Atstep 406, a current rotational position indicator bit is read from themagnetic storage medium and, at step 408, a rotational position bitsequence comprising the current rotational position indicator bit isformed. At step 410, the rotational position bit sequence is comparedwith the predetermined bit pattern and, at step 412, based on thecomparison, it is determined whether the magnetic medium is at the knownposition.

Those skilled in the art will appreciate that numerous changes andmodifications may be made to the preferred embodiments of the inventionand that such changes and modifications may be made without departingfrom the spirit of the invention. For example, it should be understoodthat the number of bit patterns is not limited to four; fewer or morecould be used. Similarly, the length of the patterns is not limited tonine bits; again, fewer or more could be used. It is therefore intendedthat the appended claims cover all such equivalent variations as fallwithin the true spirit and scope of the invention.

I claim:
 1. A method for determining a rotational position of a magnetic storage medium divided into a plurality of servo sections, the method comprising the steps of: defining a predetermined bit pattern that corresponds to a known rotational position of the magnetic storage medium; associating a single rotational position indicator bit with each said servo section such that a known bit of the predetermined bit pattern is associated with the known rotational position of the magnetic storage medium; reading a current rotational position indicator bit from a first servo section on the magnetic storage medium; reading a second rotational position indicator bit from a second servo section on the magnetic storage medium, the second servo section being adjacent to the first servo section; forming a rotational position bit sequence comprising the current rotational position indicator bit and the second rotational position indicator bit; comparing the rotational position bit sequence with the predetermined bit pattern; and based on the comparison, determining whether the rotational position of the magnetic medium is the known rotational position, wherein the rotational position bit sequence and the predetermined bit pattern each has a length of N bits, where N is defined based on a number of allowable bit errors.
 2. The method of claim 1 wherein N is at least nine.
 3. The method of claim 2 wherein N is nine.
 4. The method of claim 1 wherein the step of forming the rotational position bit sequence comprises the steps of: storing a preexisting bit sequence having a most significant bit; eliminating the most significant bit of the preexisting bit sequence to form a temporary bit sequence; and forming the rotational position bit sequence by appending the current rotational position indicator bit to the temporary bit sequence.
 5. The method of claim 4 wherein the step of forming the rotational position bit sequence is performed via a shift register.
 6. The method of claim 1 wherein the predetermined bit pattern is selected from the group consisting of 001110101, 010110011, 101001101, and
 101111011. 7. The method of claim 1, further comprising: defining a set of predetermined bit patterns wherein each predetermined bit pattern within the set corresponds to a unique rotational position of the magnetic storage medium; associating the rotational position indicator bits with the servo sections such that a known bit of each predetermined bit pattern within the set is associated with the corresponding unique rotational position of the magnetic storage medium; comparing the rotational position bit sequence with each predetermined bit pattern within the set; and based on the comparison, determining whether the rotational position of the magnetic medium is one of the unique rotational positions.
 8. The method of claim 7, wherein defining the set of predetermined bit patterns comprises: defining an allowable number of bit errors; calculating a distance between bit patterns based on the allowable number of bit errors; and determining the set of predetermined patterns so that every predetermined bit pattern in the set differs from every other bit pattern in the set by at least the calculated distance.
 9. The method of claim 8, wherein the allowable number of bit errors is two.
 10. The method of claim 8, wherein the distance is one greater than the allowable number of bit errors.
 11. The method of claim 8, wherein the set of predetermined bit patterns comprises at least one bit pattern selected from the group consisting of 001110101, 010110011, 101001101, and
 101111011. 12. The method of claim 8, wherein the set of predetermined bit patterns comprises 001110101, 010110011, 101001101, and
 101111011. 13. The method of claim 8, wherein the set of predetermined bit patterns consists of 001110101, 010110011, 101001101, and
 101111011. 14. A storage medium having a servo pattern thereon, the servo pattern comprising: a plurality of servo sectors, each said servo sector representing a portion of the magnetic storage medium; and a single rotational position indicator bit associated with each said servo sector, the rotational position indicator bits of adjacent servo sectors being a predetermined bit pattern associated with a known rotational position of the magnetic storage medium, wherein the predetermined bit pattern has a length of N bits, where N is defined based on a number of allowable bit errors.
 15. The storage medium recited in claim 14 in a method for determining a rotational position of the magnetic storage medium, the method comprising the steps of: reading a current rotational position indicator bit from the storage medium; forming a rotational position bit sequence comprising the current rotational position indicator bit and the rotational position indicator bits of the adjacent consecutive servo sectors, wherein the rotational position bit sequence has a length of N bits; comparing the rotational position bit sequence with the predetermined bit pattern; and based on the comparison, determining whether the rotational position of the storage medium is a known rotational position.
 16. The storage medium recited in claim 14 wherein bit patterns 001110101, 010110011, 101001101, and 101111011 represent four different positions of the storage medium.
 17. A method for determining a rotational position of a magnetic storage medium divided into a plurality of servo sections, the method comprising the steps of: defining a predetermined bit pattern that corresponds to a known rotational position of the magnetic storage medium, wherein the predetermined bit pattern is selected from the group consisting of 001110101, 010110011, 101001101, and 101111011; associating a single rotational position indicator bit with each said servo section such that a known bit of the predetermined bit pattern is associated with the known rotational position of the magnetic storage medium; reading a current rotational position indicator bit from a first servo section on the magnetic storage medium; reading a second rotational position indicator bit from a second servo section on the magnetic storage medium, the second servo section being adjacent to the first servo section; forming a rotational position bit sequence comprising the current rotational position indicator bit and the second rotational position indicator bit; comparing the rotational position bit sequence with the predetermined bit pattern; and based on the comparison, determining whether the rotational position of the magnetic medium is the known rotational position.
 18. The method of claim 17, wherein the rotational position bit sequence and the predetermined bit pattern each has a length of N bits, where N is defined based on a number of allowable bit errors.
 19. The method of claim 18, wherein N is at least nine.
 20. The method of claim 19, wherein N is nine.
 21. The method of claim 17, wherein the step of forming the rotational position bit sequence comprises the steps of: storing a preexisting bit sequence having a most significant bit; eliminating the most significant bit of the preexisting bit sequence to form a temporary bit sequence; and forming the rotational position bit sequence by appending the current rotational position indicator bit to the temporary bit sequence.
 22. The method of claim 21, wherein the step of forming the rotational position bit sequence is performed via a shift register.
 23. The method of claim 17, further comprising: defining a set of predetermined bit patterns wherein each predetermined bit pattern within the set corresponds to a unique rotational position of the magnetic storage medium; associating the rotational position indicator bits with the servo sections such that a known bit of each predetermined bit pattern within the set is associated with the corresponding unique rotational position of the magnetic storage medium; comparing the rotational position bit sequence with each predetermined bit pattern within the set; and based on the comparison, determining whether the rotational position of the magnetic medium is one of the unique rotational positions.
 24. The method of claim 23, wherein defining the set of predetermined bit patterns comprises: defining an allowable number of bit errors; calculating a distance between bit patterns based on the allowable number of bit errors; and determining the set of predetermined patterns so that every predetermined bit pattern in the set differs from every other bit pattern in the set by at least the calculated distance.
 25. The method of claim 24, wherein the allowable number of bit errors is two.
 26. The method of claim 24, wherein the distance is twice the allowable number of bit errors.
 27. The method of claim 24, wherein the set of predetermined bit patterns comprises at least one bit pattern selected from the group consisting of 001110101, 010110011, 101001101, and
 101111011. 28. The method of claim 24, wherein the set of predetermined bit patterns comprises 001110101, 01010011, 101001101, and
 101111011. 29. The method of claim 24, wherein the set of predetermined bit patterns consists of 001110101, 010110011, 101001101, and
 101111011. 30. A storage medium having a servo pattern thereon, the servo pattern comprising: a plurality of servo sectors, each said servo sector representing a portion of the magnetic storage medium; and a single rotational position indicator bit associated with each said servo sector, the rotational position indicator bits of adjacent servo sectors being a predetermined bit pattern associated with a known rotational position of the magnetic storage medium, wherein the predetermined bit pattern is selected from the group consisting of 001110101, 010110011, 101001101, and
 101111011. 31. The storage medium recited in claim 30 in a method for determining a rotational position of the magnetic storage medium, the method comprising the steps of: reading a current rotational position indicator bit from the storage medium; forming a rotational position bit sequence comprising the current rotational position indicator bit and the rotational position indicator bits of the adjacent consecutive servo sectors; comparing the rotational position bit sequence with the predetermined bit pattern; and based on the comparison, determining whether the rotational position of the storage medium is a known rotational position.
 32. The storage medium recited in claim 30 wherein bit patterns 001110101, 010110011, 101001101, and 101111011 represent four different positions of the storage medium.
 33. A method for determining a rotational position of a magnetic storage medium divided into a plurality of servo sections, the method comprising the steps of: defining a set of predetermined bit patterns, wherein each predetermined bit pattern within the set corresponds to a unique rotational position of the magnetic storage medium, by defining an allowable number of bit errors, calculating a distance between bit patterns based on the allowable number of bit errors, and determining the set of predetermined patterns so that every predetermined bit pattern in the set differs from every other bit pattern in the set by at least the calculated distance; associating a respective single rotational position indicator bit with each servo section such that a known bit of each predetermined bit pattern within the set is associated with the corresponding unique rotational position of the magnetic storage medium; reading a current rotational position indicator bit from a first servo section on the magnetic storage medium; reading a second rotational position indicator bit from a second servo section on the magnetic storage medium, the second servo section being adjacent to the first servo section; forming a rotational position bit sequence comprising the current rotational position indicator bit and the second rotational position indicator bit; comparing the rotational position bit sequence with each predetermined bit pattern within the set; and based on the comparison, determining whether the rotational position of the magnetic medium is one of the unique rotational positions.
 34. The method of claim 33, wherein the rotational position bit sequence and the predetermined bit patterns each has a length of N bits, where N is defined based on the number of allowable bit errors.
 35. The method of claim 34, wherein N is at least nine.
 36. The method of claim 35, wherein N is nine.
 37. The method of claim 33, wherein the step of forming the rotational position bit sequence comprises the steps of: storing a preexisting bit sequence having a most significant bit; eliminating the most significant bit of the preexisting bit sequence to form a temporary bit sequence; and forming the rotational position bit sequence by appending the current rotational position indicator bit to the temporary bit sequence.
 38. The method of claim 37, wherein the step of forming the rotational position bit sequence is performed via a shift register.
 39. The method of claim 33, wherein the predetermined bit patterns are selected from the group consisting of 001110101, 010110011, 101001101, and
 101111011. 40. The method of claim 33, wherein the allowable number of bits errors is two.
 41. The method of claim 33, wherein the distance is twice the allowable number of bit errors.
 42. The method of claim 33, wherein the set of predetermined bit patterns comprises at least one bit pattern selected from the group consisting of 001110101, 010110011, 101001101, and
 101111011. 43. The method of claim 33, wherein the set of predetermined bit patterns comprises 001110101, 010110011, 101001101, and
 101111011. 44. The method of claim 33, wherein the set of predetermined bit patterns consists of 001110101, 010110011, 101001101, and
 101111011. 45. A storage medium having a servo pattern thereon, the servo pattern comprising: a plurality of servo sectors, each said servo sector representing a portion of the magnetic storage medium; and a respective single rotational position indicator bit associated with each said servo sector, the rotational position indicator bits of adjacent servo sectors being one of a set of predetermined bit patterns, wherein each predetermined bit pattern within the set corresponds to a unique rotational position of the magnetic storage medium, and the rotational position indicator bits are associated with the servo sectors such that a known bit of each predetermined bit pattern within the set is associated with the corresponding unique rotational position of the magnetic storage medium, wherein the set of predetermined bit patterns is defined by defining an allowable number of bit errors, calculating a distance between bit patterns based on the allowable number of bit errors, and determining the set of predetermined patterns so that every predetermined bit pattern in the set differs from every other bit pattern in the set by at least the calculated distance.
 46. The storage medium recited in claim 45 in a method for determining a rotational position of the magnetic storage medium, the method comprising the steps of: reading a current rotational position indicator bit from the storage medium; forming a rotational position bit sequence comprising the current rotational position indicator bit and the rotational position indicator bits of the adjacent consecutive servo sectors; comparing the rotational position bit sequence with each predetermined bit pattern within the set; and based on the comparison, determining whether the rotational position of the magnetic medium is one of the unique rotational positions.
 47. The storage medium recited in claim 45 wherein bit patterns 001110101, 010110011, 101001101, and 101111011 represent four different positions of the storage medium. 